This invention pertains to the control of cutting depth and grade for bucket excavators of the general type shown in Satterwhite U.S. Pat. Nos. 3,896,571 and 3,974,580 and co-pending Bryan application entitled "Bucket Chain Excavator". Such excavators have a large bucket excavating means mounted on a structural main frame at the leading end of the machine. The bucket excavating means has two or more sections which are mounted on either side of extended frame members so that the bucket excavating means is the widest part of these excavators. Such machines thus have the unique capability of passing through a trench under excavation and advancing along its bottom. Closely following the bucket excavating means on the main frame is a separately mounted moldboard/skid plate assembly. The entire machine is supported on a crawler track or rubber tired undercarriage which can be raised or lowered. This movement adjusts the cutting depth of the excavating means relative to the ground contact plane of the undercarriage. The moldboard blade breaks up uncut material left between excavating means sections and scrapes the bottom of the cut clean, crowding loose materials forward to be picked up by the buckets. The bucket excavating means operates in an undercutting manner and these materials are carried, along with the freshly dug material, to be discharged onto a conveyor.
In normal, day-to-day operation, the excavator is routinely required to "ramp-down" to a predetermined cutting depth, to "hold grade", making any required changes thereto, and then to "ramp-up", back to a higher elevation.
The prior art, as typified by U.S. Pat. No. 4,069,605, discloses only the raw capability for digging downwardly or upwardly inclining ramps or making level cuts, but has failed to teach any means or method for doing these things in a stable, predictable, accurately controllable manner.
Prior to the present invention, grade control or the selection and regulation of depth of cut of the digging wheel, has been a difficult and uncertain task at best, such that even the most skilled operators have been unable to achieve consistently acceptable results. The effect of any control input has not been perceptible until the machine has advanced significantly, so that an action that seems appropriate frequently proves to be wholly incorrect.
This difficulty of operation has been compounded by interrelated control variables. Increasing the depth of cut, for instance, has been accomplished by optionally raising the front suspension point of the supporting undercarriage, or by similarly lowering the rear suspension point, or by some combination of these adjustments. An alternate means for increasing the depth has been raising the moldboard/skid plate, allowing the digging wheel to dig more deeply. In a like manner, the reverse of each of these actions might be used to reduce digging depth. It is characteristic of these machines that any deviation from grade will increase at an accelerating rate as the machine advances. This rate of deviation is determined by the relative elevations of the digging wheel and the moldboard/skid plate to the front and rear suspension points. In addition to the aforementioned delayed response, the operator has also been hampered by an absence of visual clues as to the relative positions of these several grade determining elements.
The objects of the present invention are to achieve a manageable grade control system by:
1) Maintaining the position of the grade determining elements in a disciplined, stable relationship. PA1 2) Restricting grade control to adjustment of a single grade determining element. PA1 3) Providing the operator with grade control information in a usable display format.
The present invention first recognizes that grade control of the excavator can be manageable only when the machine operates in a disciplined manner and has an inherent capability for making smooth, predictable grade transitions.
The invention comprises modifications to the excavator which incorporate the above characteristics into the machine operation. This is achieved by placing the bottom of the digging wheel, the blade edge, and the ground support points, i.e., the grade determining elements, in a flat plane as the machine is when configured to cut a constant grade, and then causing this plane to become a cylindrical surface of proportionately decreasing radius as the digging wheel is raised or lowered to initiate a change of grade. In effect, the constant grade configuration "flat plane" is a cylindrical surface of infinite radius.
This cylindrical surface is mutually tangent to the digging wheel and to the front and rear ground support elements and is in fact, the grade path for the excavator. The case of a track laying undercarriage differs somewhat from that of a wheeled system in that, the ideal tangency is at the front and rear track sections for upward, or concave, transitions and at the track section just behind the dynamic balance point of the excavator for downward, or convex, transitions. The cylindrical surface of transition becomes mutually tangent to the initial grade produced by the excavator and the new grade.
The moldboard/skid plate assembly has heretofore typically been raised or lowered at the operator's discretion in an effort to hold it at or very near grade elevation at all times as described by Satterwhite U.S. Pat. No. 4,069,605, or it has been positioned by a linkage means of arbitrary geometry.
Prior art has considered the moldboard to be positioned more or less in coordination with the digging wheel position, or by the operator, uncoordinated with other functions. The foregoing are shown in Satterwhite U.S. Pat. No. 4,069,605. The contribution made by the moldboard/skid plate to the function and control of the excavator has heretofore been neither fully understood nor disclosed.
In the present invention, elimination of moldboard blade edge position as a variable is a fundamental step towards grade control simplification in the context of the present invention. Elimination of that variable, along with fixing either the front end or the rear end undercarriage elevation, leaves only the elevation of the free end of the undercarriage as a grade controlling variable. This single variable feature reduces the operator's work load to a minimum and permits grade related information to be organized and displayed in a usable format.